Environmental factors that cause genetic damage are a major public health concern. Methods to assess risk of exposure to potentially mutagenic agents have been developed, but fall short in several ways. There is a need for a method that is capable of detecting a variety of genetic changes (including homologous recombination) at the same level (individual cells) and ina the same context (intact organs) as they occur in humans. We propose to develop such a system by using a transgene that encodes an enzyme (human placental alkaline phosphatase, PAP), the activity of which can be detected in individual cells of a tissue with a histochemical stain. To allow detection of transition mutations, the PAP transgene has been altered at a single base (an A was changed to a G), which prevents production of PAP activity. The mutant PAP gene has been inserted into the genome of mice. The tissues of these animals are expected to not stain for PAP activity, except for rare cells that contain a PAP gene that has sustained a reversion mutation. Similarly, a second transgenic mouse has been made that is designed to detect frameshift mutations. This animal carries a mutant PAP gene made by insertion of a G residue into a run of G residues near the 5' end of the open reading frame encoding PAP. We propose to construct two additional types of transgenic animals designed to allow detection of homologous recombination. One such animal will detect interstitial deletion events, the other will detect mitotic recombination. These four lines of transgenic mice will be used to define the spontaneous occurrence of mutant cells in various mouse tissues. We will next determine the effects of model mutagens in these animals. Then, the transgenic animals will be crossed with other mouse strains that are cancer prone due to genetic defects, such as lack of p3, or PMS2. Through studying spontaneous and induced genetic change ina the transgenic mice, we will explore the relationship between mutational load and tumor incidence, and test the hypothesis that genetic change is more frequent in tumors and precancerous cells than in normal cells of the body.